With VA, you can buy Y(X) which is very good.
I have now used that row 3 many times, pity QF don't have this, but of course, they do have comfort seat which is a slight plus.
VA Y(X) is where you pay a bit over the economy and even on the 737, you have stretch in front of you, from row 3 to the divider, granted that the meal tray is in the arm rest, making that immobile, but still, for $49/$59 above the economy fare, you have a bit of stretch factor.
OP, if I were in your position, I would for the flight PER - SYD, I would get the QF 737, or that elusive QF 330 (which might be swapped for a 737 anyways) in J so that have access to the QFd PER J lounge, and on the way back from SYD, buy QF comfort seat, even on the 737, so this gives you the seat next as empty, if its a flight departing at 20.30, with a comfort seat buy you still can lift the arm rest with a comfort seat with the seat next to you as empty.
Edit: the trouble with upgrades, is that you to buy a QF flex Y fare, as the cheaper fares are non upgradable, so this is why I say for PER - SYD, if you see availability is to use points to grab that J seat ASAP, try to get to the J lounge at 18.30/19.00 this is when most likely good hot food will be available.
Get there to the lounge too late and you only get maybe nuts and grog.
SYD - PER, day or night, you will be in Y with a space next to you, but the SYD J lounge is a bit oldish now. PER lounge is newish.